The present invention relates to shoe soles and shoe sole components and to methods of manufacturing shoe soles and shoe sole components. More particularly, the invention relates to a flexible high polymer resin shoe sole or sole component and a method of thermoforming a shoe sole or sole component.
A shoe generally consists of two basic parts, an upper and a sole. The upper is generally designed to enclose and provide cushioning to the foot. The upper also typically includes an insole to provide initial support and cushioning to the bottom of the foot. The upper is attached to a sole which provides additional protection and cushioning primarily to the bottom of the foot. The sole also imparts stability to the shoe.
The increase in demand for shoes for sports and outdoor activities such as walking, running, hiking, tennis, basketball and numerous other high activity sports has prompted many advances in shoe design to provide improved protection and comfort to the feet, ankles, legs, hips, etc. Efforts to improve shoes have centered on decreasing shoe weight and improving cushioning, flexibility, and stability. In addition, shoe soles are desired with improved memory, shock dispersion capabilities and energy return.
The effort to design improved shoes has prompted increased study of foot dynamics in general and the study of foot dynamics as it relates to particular activities such as running, basketball and other specific activities. The combination of research and research responsive shoe design has resulted in shoes designed for specific activities. For example, the pressures exerted by a foot on a sole when walking are different from the pressures exerted by a foot when running, or when playing tennis, etc. Accordingly, the modern sport shoe design takes into account the specific requirements of shoes used for particular activities. In addition, modern sport shoe design attempts to take into account the specific needs of the individual, such as weight, foot width and other individual characteristics such as pronation and supination. Thus, general considerations such as shoe weight, cushioning, flexibility and stability are taken into account in designing sport shoes for particular activities and individual needs. Although the functional characteristics of the shoe are of primary importance, other factors such as cost and appearance of the shoe must be taken into account for full consumer satisfaction.
Sport shoe refinements have concentrated particularly on the sole. Sport shoe soles typically have two components, the midsole and the outsole. The outsole is the ground-contacting portion of the sole and provides traction and protection to the remainder of the sole. Outsoles, accordingly, are composed of durable materials, such as rubber, which provide both traction and high abrasion resistance. The midsole contributes to foot stability and is the primary shock absorption member of the sole. The midsole is composed generally of a softer, more flexible material than the outsole. Since the midsole is important to such factors as stability and shock absorption, the design of the midsole has received considerable attention by sport shoe manufacturers.
Typically, midsole construction centers around plastics expanded into foams which are then shaped in a number of ways to accommodate a shoe upper. The foam midsole is then covered with a more durable sheet of outsole material, usually rubber, to provide the sole with adequate abrasion resistance and traction. Attaching an outsole to a foam midsole is generally a labor-intensive process. For example, attaching a rubber outsole to a midsole requires abrading the surface of the midsole, washing the surface with a solvent, layering both the midsole and outsole surfaces with cement where they are to be joined, then activating the treated surfaces, usually with heat. This is followed by touch-up and decoration processes.
A foam midsole material by itself is generally inadequate to provide the stability and cushioning demanded for modern sport shoes. The foams used in current soles have insufficient external surface tension by themselves to provide the required stabilizing forces in response to pressures exerted on a sole. This is especially true with extremely low density foams employed to minimize weight. Moreover, current foam midsoles quickly lose their ability to adequately cushion, often after as little as 20% of the shoe life.
The problems of stability and cushioning associated with the use of foam midsoles has prompted several approaches for increasing stability and prolonging the cushioning properties of midsoles. Efforts to improve stability have centered on the use of inserts of denser, more rigid materials than the main midsole component, such as dense foam plugs or solid thermoplastic inserts. These are either inserted directly into the foam midsole component before curing or cemented into place afterwards in another labor-intensive process. Efforts to improve cushioning while maintaining adequate stability have centered on the use of flexible thermoplastic inserts and liquid or gas filled inserts. These inserts also are generally encapsulated in the main midsole component. Thus, modern sole design has centered on constructing soles having varying degrees of flexibility in selected areas of the sole. For example, inserts can include foamed plugs of material harder or softer than the main midsole component. As stated, the inserts are typically encapsulated in the midsole material to provide areas in the midsole of lesser flexibility where increased stability is desired, and areas of greater flexibility where increased cushioning is desired. Other approaches include the use of resilient spheres embedded in the midsole by casting or injection molding the midsole around the spheres. Trampoline devices incorporated into the heel region of shoes have been attempted, but cost and appearance factors have limited the use of this concept. Various gels and gases have also been incorporated into midsoles in an attempt to enhance and prolong cushioning and energy return. However, soles incorporating gels or gases such as air are costly to manufacture and relatively unpredictable in their functional characteristics.
Midsoles have also been constructed of a shell of a thermoplastic elastomer designed to encapsulate and protect low density synthetic foams such as polyether, polyurethane or polyester polyurethane. Increased rigidity along the periphery of the sole is provided by convolutions in the shell material along the edge of the sole. Plugs of denser foam are still incorporated into the main foam component where more rigidity in the inner foam component is desired. Moreover, such shells are generally made by blow molding the thermoplastic material. Blow molding involves costly molds, which limit the size and number of design changes. Additionally mold costs become a significant factor when the number of shoe sizes for each shoe model is taken into account. This is reflected in higher sole unit costs. Thus, shoes manufactured by blow molding techniques are limited to higher priced shoes although the gain in performance over traditional sport shoes employing foam midsoles is not particularly significant since the interior of the shell sole is still composed of foam which breaks down and loses its cushioning effect in a relatively short period of time. Because of the cost involved in manufacturing such soles, their use has been limited primarily to heel components. Accordingly, shoe sole components are desired having improved performance and cost effectiveness.